The forces necessary to guide a vehicle along a railway track occur in the area of contact between wheel and rail, the wheel-rail contact. These forces are however also responsible for negative effects on the rails and wheels. Thus tangential forces, which are always associated with sliding effects and thus with friction, cause profile wear by removing material. Furthermore the forces attacking wheel and rail, with a sufficiently high level of force, cause fatigue in the material, resulting in Rolling Contact Fatigue (RCF). This results in fine cracks in the rail and/or in the wheel for example. Head checks represent a typical form of damage caused by this fatigue on the rail surface. Cracks can occur in the wheel below the surface, grow outwards and lead to larger pits. However the cracks can also occur on the surface per se, grow deeper and likewise lead to material failures, as also occurs for example in the known phenomenon of the herringbone pattern. With surface-initiated cracks there is the effect of the cracks being partly removed again by the said profile wear, with the result that a certain degree of profile wear can sometimes be desirable. As well as the running surface damage mentioned, a variety of further forms of damage, such as flat spots, material removal, cracks across running surfaces etc., also occur.
Thus particular safety relevance is attached to the wheel-rail contact, with high-speed trains too for example. Irregularities in the wheel-rail contact, caused for example by serious damage to a wheel, can lead to major consequential damage through to derailment. But light damage such as fine cracks can also lead to great difficulties, since they make maintenance work necessary and can thus result in high costs and delays in rail traffic.
A range of mechanical devices for guiding a rail vehicle on a track are thus known. Many of the known systems are based on the fact that, when a curve is negotiated, the radial positions of the wheels in the track are optimum in order to reduce the forces acting on the independent wheel sets or wheel sets of a bogie for a rail vehicle. Thus, the argument goes, the friction and therefore also the profile wear in rail-wheel contact can be reduced.
For example EP 0 600 172 A1 describes a bogie for rail vehicles in which the wheel sets are turned outwards when negotiating a curve by means of force-controlled actuators. In this patent however a radial position of the wheel sets relative to the track is not realized, but only the angle between wheel set and bogie frame is adjusted according to the radial position. Thus although favorable wear behavior obtains in many operating states, this does not correspond to the optimum.
DE 44 13 805 A1 discloses a self-steering, three-axle bogie for a rail vehicle in which the two outer wheel sets are provided with a radial control and the inner wheel set is able to be moved lateral to the direction of travel by an active actuator. The lateral forces on the outer wheel sets are reduced by this—with a suitable application of the active actuator a third of the centrifugal force acts on each wheel set. Thus all three wheel sets are included for control during negotiation of a curve, the alignment of the wheel sets to the center of the curve is improved.
A further method of this type can be found in the applicant's patent EP 1 609 691 A1.
Common to all these methods is that they aim to minimize the friction and thus the profile wear in wheel-rail contact. In these methods the position of the wheels relative to the track is influenced so that sliding effects at the point of contact are avoided or minimized. However rolling contact fatigue also causes damage to rail and wheel. To rectify this damage a certain degree of friction can even be desirable, since cracks occurring in the material can be removed at the surface by said wear. A minimum of friction thus does not always correspond to an optimum loading ratio in wheel-rail contact.
The applicant's unpublished application A942/2007 “Verfahren zur Minimierung von Laufflächenschäden and Profilverschleiβ von Rädern eines Schienenfahrzeugs” (Method for minimizing running surface damage and profile wear of wheels of a rail vehicle) discloses a model-based method for optimizing the wear behavior of rail vehicle wheels. In this method the running surface wear is optimized by means of actuator-controlled displacement (lateral displacement of the independent wheel set or wheel set axles or angular displacement between the axles) on the basis of measured values determined.
In these cases various bogie geometries are provided, which makes the angular and lateral displacement of the axles possible. The so-called lateral actuators required for this render the construction of the bogie extraordinarily difficult.